This invention relates generally to the chemical process industries, and more particularly to an improvement in waste gas abatement systems for a nitric acid producing plant.
The present invention removes solid ammonium nitrate particulate matter from a flow of off gases or waste gas from an industrial process such as the production of nitric acid,
In the nitric acid production process, the waste gases produced contain large amounts of nitrogen oxides gases. To meet air pollution standards, a substantial portion of these nitrogen oxides gases must be removed from the waste gas flow, prior to exhaust thereof into the atmosphere. Accordingly, a nitrogen oxides abatement system has heretofore been suggested, which system receives the waste gas flow, and generally functions to achieve contact of the waste gases with solutions of water, ammonia and nitric acid of controlled pH, to lower the level of nitrogen oxides gases in the gas stream to an acceptable level. Broadly speaking, this abatement process causes a substantial portion of the nitrogen oxides gases to react with the ammonia to form a solution of ammonium nitrate in water which may then be removed for other uses as desired. However, it has been found that the process also results in the formation of small particles of solid ammonium nitrate in the gas stream which do not go into solution. These particles of ammonium nitrate in the gas stream cause problems of build up and plugging downstream in the system as described below. It has been found that reducing the amount of ammonia used in the nitrogen oxide abatement process substantially eliminates this formation of ammonium nitrate particles in the gas stream. However, such reduction of ammonia also reduces the effectiveness of the nitrogen oxide abatement system, resulting in an unacceptably high amount of nitrogen oxides gases being exhausted to the atmosphere.
Specifically, with regard to the downstream pluggage problem, a typical nitric acid plant utilizes the waste gas flow in a power recovery compressor for providing compressed air to the nitric acid production process. Accordingly, the waste gas stream is fed through a heat exchanger counter to a flow of hot gas, and the heated gas is then utilized to drive a turbine which powers the compressor. Consequently, the presence of ammonium nitrate particles in the gas stream causes a build up and eventual plugging in the elements of this power recovery system. Removal of most of this particulate matter from the gas stream by the present invention eliminates such build up and pluggage, thus eliminating costly plant shut downs for cleaning the affected power recovery system components.